1. Field of the Invention
The present invention relates to a system which uses a cathode ray tube (CRT), and more particularly, to an apparatus and method for generating a voltage required for driving a CRT.
2. Description of the Related Art
In general, an apparatus for generating a voltage for driving a CRT provides a soft start function to prevent the circuits in the CRT from being damaged when a CRT driving voltage which drives a deflection yoke that generates horizontal deflection current in the CRT is abruptly applied.
When a conventional CRT driving voltage generating apparatus is integrated, most of the devices used for performing the soft start function must be separately installed on the outside of an integrated circuit. To overcome this problem, in one conventional CRT driving voltage generating apparatus, devices for performing the soft start function are realized by a high-cost bipolar process. As a result, this conventional CRT driving voltage generating apparatus can be integrated with the soft start devices for performing the soft start function. However, this conventional apparatus is high in cost.
Also, in the conventional CRT driving voltage generating apparatus, when the frequency of a horizontal synchronization signal is abruptly changed from a high frequency to a low frequency, a horizontal deflection system circuit may be damaged, or may not operate normally. This is because a CRT driving voltage is maintained at a higher level than required until the CRT driving voltage generating apparatus generates a stable CRT driving voltage, when the frequency of the horizontal synchronization signal is abruptly changed. This is the reason that the intensity of the CRT driving voltage is greater when the horizontal synchronization signal has a high frequency than when it has a low frequency.
An object of the present invention is to provide a CRT driving voltage generating apparatus which can be realized and integrated at a low cost, and can prevent breakdown and malfunction of circuits by actively tolerating a change in the frequency of a horizontal synchronization signal.
Another object of the present invention is to provide a CRT driving voltage generating method performed by the CRT driving voltage generating apparatus.
To achieve the first object of the invention, there is provided an apparatus for generating a cathode ray tube (CRT) driving voltage for driving a deflection yoke for generating a horizontal deflection current in a CRT. The apparatus includes a first comparator for comparing a reference value with the peak value of a fly back pulse. A stability controller shifts the level of the result of the comparison using a current mirroring technique and outputs a compared signal having a shifted level. A second comparator compares the compared signal with a sensing signal to obtain a reset signal. A pulse width modulated signal generator outputs a pulse width modulated signal having a level which is determined in response to the logic level of the reset signal and the logic level of a clock signal locked to a horizontal synchronous signal. A driving voltage generator generates the CRT driving voltage in response to the pulse width modulated signal. A voltage sensor senses the level of the CRT driving voltage to generate the sensing signal. A horizontal output unit generates the fly back pulse having an amplitude which is determined in response to (i) the frequency of a horizontal output signal when the horizontal output signal is locked to the horizontal synchronous signal, and (ii) the level of the CRT driving voltage.
In one embodiment, the apparatus of the invention also includes a frequency sensor for sensing the variation of the frequency of the horizontal synchronous signal and outputting a control signal in response to the sensed variation. The stability controller shifts the level of the result of the comparison in the first comparator in response to the control signal and outputs the compared signal having a shifted level. In one embodiment, the frequency sensor includes a phase locked loop for locking a self-oscillated frequency to the frequency of the horizontal synchronous signal and generating the control signal indicating whether the self-oscillated frequency has been locked to the frequency of the horizontal synchronous signal.
In one embodiment, the first and second comparators, the stability controller, the pulse width modulated signal generator and the voltage sensor are integrated into a single chip. In another particular embodiment, the single chip also includes the frequency sensor.
To achieve the second object of the invention, there is provided a CRT driving voltage generating method. In accordance with the method of the invention, it is continuously determined whether power has been applied to the apparatus for generating a CRT driving voltage. If the power has been applied, the difference between a reference value and the peak value of a fly back pulse is shifted using a current mirroring technique. The CRT driving voltage is generated using a signal having a level which is a shifted difference, wherein the amplitude of the fly back pulse is determined in response to (i) the frequency of a horizontal output signal when it is locked to a horizontal synchronous signal, and (ii) the level of the CRT driving voltage.